body force and its direction

[aradermacher]

The sign of the body force might be unclear. E.g. in simple_cube the body force is defined positive in positive z or y direction. But the gravitation would work in the opposite direction.

• add interface function in experiment base class defining the direction of the gravitation (to be fone in each child)
• implement the create_body_force fct globally in the base class once for all